The world's best focusing aid
Pavel Bahtinov, a Russian amateur astronomer, came up with this mask (how... I don't know) that makes any previous focusing aid obsolete. I don't care whether it's a Ronchi screen, knife edge, software... doesn't matter; the mask knocks them all out. So... how does it work.
'The Mask' as I will refer to it creates a series of three diffraction spikes; two short ones that form an X and a long one that bisects the X. Now the neatness... the long one ONLY bisects the X if the optics are in perfect focus. If not... then the long spike is displaced to one side or the other. See the images below.
I had my masks made by Lazer-Tech, a London, ON company that does laser cutting very reasonably. My small mask for the SV102ED was $20Cdn and the large mask for the C8 (see below) was $25Cdn. The are cut from 3mm plastic that is TOUGH. The cutout edges are sharp and clean: they work perfectly and make achieving critical focus a SNAP.
Lazer-Tech needs an Adobe Illustrator file to work with (they can use other formats but the AI files are by far the best. You can either use the online Bahtinov Mask Generator or you can download a ready-made AI file from my site. If you use the generator you'll need to convert the file from an SVG to an AI. All of my files have at least a 40mm rim around the outside to make mounting easy. I mount mine with two silicone cones that came with a mask that I bought, but you can figure out the best way for your setup.
Contact Mark at Lazer-Tech (information on their page) to answer any questions.
C8 with the Mask installed
Image of the Pleiades taken through the SV102ED after focusing using the Mask
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